Titania (or titanium dioxide) is a well-known white inorganic pigment, and two main processes are used to prepare titania on a commercial scale, namely the so-called “chloride process” and the so-called “sulphate process”.
In the “chloride process”, titanium-containing raw materials are chlorinated at 700-1200° C. Titanium tetrachloride is separated from the other chlorides by distillation. The titanium tetrachloride, optionally after further purification, is burnt with an oxygen containing gas at temperatures between 900-1400° C. to form titania.
In the “sulphate process”, the titanium-containing raw material is dissolved in concentrated sulphuric acid at 150-220° C. Removal of insolubles and precipitation of iron sulphates results in a concentrated titanyl sulphate solution. Relatively pure titania dihydrate is precipitated by hydrolysis of the sulphate solution at about 100° C. The remaining impurities, especially metal sulphates, are largely removed in further purification stages, especially by washing with diluted acid or by bleaching. The hydrate can be filtered until a titania content of 30-40 wt % is obtained. Then the filtrate is dried, calcined, ground and further treated, depending on the type of application.
Besides the use of titania as a pigment, it has other applications. One is as use as a catalyst itself in certain chemical reactions. Another application is the use as a catalyst carrier or support.
One area of use of titania-based catalyst carriers is the catalytic preparation of hydrocarbons from a gaseous mixture comprising carbon monoxide and hydrogen, generally known as the Fischer-Tropsch process.
The Fischer-Tropsch process can be used for the conversion of hydrocarbonaceous feed stocks into liquid and/or solid hydrocarbons. The feed stock (e.g. natural gas, associated gas, coal-bed methane, residual oil fractions, biomass and/or coal) is converted in a first step into a mixture of hydrogen and carbon monoxide (this mixture is often referred to as synthesis gas or syngas). The synthesis gas is then fed into a reactor where it is converted in a single step over a suitable catalyst at elevated temperature and pressure into paraffinic compounds ranging from methane to high molecular weight modules comprising up to 200 carbon atoms, or, under particular circumstances, even more.
Catalysts used in the Fischer-Tropsch synthesis often comprise a titania based support material and one or more metals from Group VIII of the Periodic Table, especially from the iron group, optionally in combination with one or more metal oxides an/or metals as promoters. Particular interest has been given to catalysts comprising cobalt as the catalytically active component, in combination with one or more promoters selected from zirconium, titanium, chromium, vanadium rhenium, platinum and manganese. Such catalysts are known in the art and have been described for example, in the specifications of International Patent Application No. WO A-9700231 and United States Patent publication No. U.S. Pat. No. 4,595,703.
There is a continuous interest in more efficient ways to prepare catalysts carriers and/or catalysts, especially titania based carriers and catalyst. In addition there is a continuous interest in the preparation of catalysts having increased strength, activity and/or selectivity. As discussed above, titania catalyst carrier is either prepared by a high temperature process or in a relatively low temperature process followed by calcination. In a second step the carrier material is admixed with catalyst materials, shaped and dried/calcined. Calcination is in particular needed to make strong catalyst particles.
It has now been found that titania catalyst (carrier) and titania-supported catalysts can be obtained by the hydrolysis of a suitable titanium compound, followed by filtration to obtain a wet filtercake of titania and use of the wet filter cake in the catalyst (carrier) preparation.